Home  ›  Blog  ›  Busbar Trunking Specification

What Should a Busbar Trunking Specification Include?

By DHYN Engineering Team  ·  September 30, 2026

Quick answer: A busbar trunking specification should name the product standard (IEC 61439-6, or UL 857 in North America), the rated current and voltage, the short-circuit withstand in kA, the IP rating, the conductor material and plating, the neutral size, the tap-off requirements and the temperature rise class. Then finish with the line that protects you: type test reports must be submitted with the bid.

Most busway specs we receive are either one line ("1600A busbar trunking, IP54") or a 40-page novel copied from another project. Both cause trouble. Here is what a workable specification actually needs, in the order we read them as a manufacturer.

Start with the standard and the ratings

One sentence fixes the whole playing field: "Busbar trunking shall be type tested to IEC 61439-6." That single reference drags in verified temperature rise, short-circuit withstand, dielectric and protection tests, and it kills the old partially-type-tested loophole. Our IEC 61439-6 explainer covers why that clause matters. Then state rated current per run, rated voltage (400/415 V, or 690 V where applicable), frequency, and the number of conductors: three-phase plus neutral, plus how the earth is provided.

Short-circuit and temperature rise

Write the fault level you calculated, not a wish. Something like "Icw 50 kA for 1 second, Ipk 105 kA" is a real requirement; "adequate short-circuit strength" is not. If you have not worked out your fault level yet, our short-circuit rating article shows the quick method from transformer kVA and impedance. For temperature rise, referencing IEC 61439-6 is usually enough, since its 70 K terminal limit comes with the territory. Add an ambient temperature line if the duct will see anything above the standard 35°C average, because that triggers derating, which we cover in busway thermal derating factors.

Enclosure, conductors and neutral

State the IP rating per zone, not one blanket figure: IP54 for indoor dry areas is standard, outdoor or washdown sections need IP65 or above. Our IP ratings guide helps you pick. Then the conductor: copper or aluminum, tin plated or bare, and the housing finish, galvanized or epoxy coated. Finish with the neutral. If the loads include LED, VFD or IT equipment, write "neutral 100% of phase cross-section" as a minimum, and read bus duct neutral sizing before you leave it at 50%.

Tap-offs and accessories

This is where specs go vague and projects go over budget. List the tap-off boxes with ratings, protection type (fuse switch or MCCB), and quantity per floor or per run. Say whether units must be hot-swappable and whether metering is required. Name the accessories you actually need: flanged ends for transformer and switchboard connections, expansion fittings on long runs, fire barriers at slab and wall crossings. Each of those has a cost, and each one left out of the spec comes back as a variation order.

The documents to demand with the bid

Three lines that separate real manufacturers from resellers: type test reports to IEC 61439-6 from a recognized lab, a routine test certificate per shipping section, and installation manuals with joint torque values. Add "deviations from this specification shall be listed in the offer" and you will finally be able to compare bids line by line.

Mistakes we see in tender documents

Copying a competitor's catalog clause that only one brand can meet, which poisons the tender. Specifying the duct but forgetting the tap-offs, then discovering they cost 30% of the package. And writing "as per site requirement" for anything measurable. A spec is a measuring stick. If you cannot reject a bid with it, it is decoration.